1. Field of the Invention
The present invention relates to an optical element where light with a wide wavelength range including blue light to near infrared ray enters. Further, the invention relates to the optical element which multiplexes blue light, red light and near infrared ray whose incident directions are different from one another, and an optical pickup device using it.
2. Description of the Related Art
Optical disc devices which reproduce or record information from/into optical discs such as CD or DVD read and write signals using light with wavelengths different according to media. U.S. Pat. No. 6,005,835 discloses a constitution in which one optical pickup device reads various types of optical discs.
This optical pickup device has a plurality of light sources which emit light beams with different wavelengths. As the light beams emitted from the light sources, red light of 650 nm band (630 to 690 nm) and near infrared ray of 780 nm band (740 to 820 nm) are used to read DVD-ROM, CD-ROM, and the like. When the light source according to an optical disc is driven and light with predetermined wavelength is emitted to the optical disc, the light reflected by the optical disc is captured by a light receiving element. As a result, even if different types of optical discs are used, signals according to wavelengths for used optical discs can be read.
Emitted light from the light sources enters a prism from directions perpendicular to one another. A dichroic film which reflects or transmits the light according to wavelengths is provided to the prism, and the incident light is emitted to one direction by the dichroic film. As a result, even if different types of light sources are driven, the light beams are multiplexed so as to be capable of being emitted to the optical discs.
In recent years, since the optical discs as recording media have high density, in addition to the red light and the near infrared ray, blue light of 410 nm band (390 nm to 425 nm) is used as emitted light from the light sources. As a result, Blu-ray Discs, HD DVD and the like can be read. At this time, an anti-reflection film which prevents blue light from being reflected is provided to the incident surface for the blue light in the prism which multiplexes the light beams. An anti-reflection film which prevents red light and near infrared ray from being reflected is provided to an incident surface for the red light and the near infrared ray whose wavelengths are longer than that of the blue light. Further, an anti-reflection film which prevents the blue light, the red light and the near infrared ray from being reflected is provided to an emission surface of the prism.
FIG. 8 illustrates reflection characteristics of the conventional anti-reflection film which prevents the reflection of the blue light, the red light and the near infrared ray provided to the emission surface of the prism. Reflectance (unit: %) is read along an axis of ordinates, and wavelength (unit: nm) is read along an axis of abscissas. The anti-reflection film is formed by laminating optical thin films, and its constitution is shown in Table 1. In the drawing, T1 designates a wavelength range of the blue light, and T2 designates a wavelength range of the red light and the near infrared ray. In Table 1, H4 is a product made by Merck KGaA, and a mixture of TiO2 and La2O3.
TABLE 1LayerMaterialRefractive indexFilm thickness nm8MgF21.385109.667H42.1530.016MgF21.38525.815H42.1557.444MgF21.38523.943H42.1529.112MgF21.38510.001Al2O31.6283.13SubstrateGlass1.52H4 is a mixture of TiO2 and La2O3
In the anti-reflection film shown in Table 1 and FIG. 8, the reflectance of the blue light, the red light and the near infrared ray is about 0.4%. The light source which emits the blue light has, however, small output, and when the reflectance of the anti-reflection film is about 0.4%, a light amount which is necessary for reading an optical disc cannot be occasionally secured due to loss. For this reason, there is a problem that reading error of an optical disc occurs. Moreover, when the reflectance of the blue light, the red light and the near infrared ray is reduced, there is another problem that a number of laminations of optical thin films increases, and thus the costs of the prism and the optical pickup device increase.